Tubular metal products are widely used in the automotive industry. In particular, metallic tubes comprise a principal part of vehicular exhaust systems which carry exhaust gases from the vehicular engine to a safe and convenient location from which the exhaust gases may be dispersed. Rectangular metal tubes also are employed to manufacture frames for trucks and other vehicles.
Tubes incorporated into a vehicle generally must be bent at several places along their lengths to avoid or to meet other parts of the vehicle. For example, tubes which carry exhaust gases from the engine typically undergo several complex bends within the engine compartment to bypass other engine components and accessories. Two or more exhaust pipes on a vehicle may bend to join with one another, and then may bend several more times to avoid the passenger compartment, an axle, a gas tank, a trunk or the like. Tubes used in frames must similarly be bent to avoid or precisely mate with other structural components on the vehicle.
Tubes typically are bent with a preprogrammed automatic bending apparatus. The bending apparatus typically will include gripping means for gripping the trailing end of the tube. The gripping means is operative to selectively advance the tube preprogrammed distances in an axial direction, and to selectively rotate the tube preprogrammed amounts about the initial longitudinal axis. The bending apparatus further includes a bending die, a portion of which defines an arc of a circle. More particularly, the arced portion of the bending die is further configured to define a groove dimensioned to receive the outer surface of the tube. For example, the groove in the arced portion of the bending die will typically be of generally semicircular configuration with a radius substantially equal to the outside radius of a circular tube to be bent. A bending die for rectangular tubes may have a corresponding rectangular groove. The bending die is mounted in the apparatus for preprogrammed rotation about a point coincident with the center of rotation of the arced outer surface.
The bending apparatus further includes a clamp die which is dimensioned and configured to securely hold a portion of the tube against the bending die. The clamp die is also mounted to the apparatus for rotation about the center of rotation of the arced portion of the bend die.
The bending apparatus further includes a pressure die which initially is aligned with the clamp die and is urged tightly against the tube. Unlike the clamp die, the pressure die does not rotate around the center of rotation of the arced portion of the bend die.
In operation, the tube to be bent is axially advanced to a preselected position relative to the bend die and is securely held against the bend die by both the clamp die and the pressure die. The clamp die and the bend die will then rotate a preprogrammed amount about the center of rotation of the arced portion of the bend die, while a movable section of the pressure die generally will follow the arc length in a controlled manner. This rotational movement of the clamp die and bend die will cause the tube to be bent an angular amount substantially equal to the degree of angular rotation by the clamp die and bend die. After a bend of a predetermined amount is completed, the tube will be released, and the clamp die, bend die and pressure die will return to their initial position. Simultaneously, the tube will be moved by the gripping means both axially and rotationally into position for completing the next bend. For most automotive uses, each tube will receive several successive bends of different angular amounts and disposed at different angular alignments relative to one another.
Certain bending devices will further include a mandrel within the tube to ensure a smooth bend, a booster to move the pressure die tangentially to feed tubular material into the bend and prevent excessive thinning, and a wiper die to minimize wrinkles on the inside portion of the bend.
There are many programmable bending devices available which will accurately advance the tube both axially and radially toward the bend die and which will accurately complete preprogrammed movements of the clamp die and bend die. Despite this accurate performance of the available bending equipment, there are often substantial differences among the bent tubes produced by the devices. In some situations, minor variations from one bent tube to the next can be tolerated. In other situations, however, even these minor variations from one part to the next create problems. For example, many new car assembly processes are being automated, and the automated assembly equipment or robots require substantial uniformity from one part to the next. This applied to both new car exhaust systems and to tubular frames for vehicles. In other situations, the close proximity of the tubular product to other vehicular components provides little room for variation. This is particularly true for exhaust systems which, by definition, carry heated exhaust gases. It is often essential to carefully control the spacing between the heated exhaust pipes and adjacent components of the vehicle, such a floorboards, fuel lines and hydraulic lines for brakes or steering. The ability to accurately and consistently produce bent tubular members within the precise specifications often must be carefully documented. The tube manufacturer generally must follow and document a particular statistical produce control (SPC) method.
In view of the greater accuracy being required for tubular products, the reasons for differences between successive parts have been studied. It has been found that one reason for variations from one bent pipe to the next is attributable to variations in the metallurgical characteristics plus material thickness and/or dimensional variations of the tubular products. More particularly, metallurgical variations will cause successive tubular products to respond differently to the forces exerted by the bending apparatus. Thus, although virtually all tubes exhibit spring-back upon release of the clamp die, the amount of spring-back can vary substantially from one tube to the next. Furthermore, it has been found that the rapid movement of the portions of the tube that have already passed through the bending apparatus will create additional rotational moments that may cause other unanticipated bends in the tubes, the magnitude of which will depend upon the length and mass of the tube, the speed at which the bending apparatus is operating and the metallurgical and dimensional characteristics of the particular tube being bent. These variations may compound one another along the length of the tube to yield substantial differences between the specified and actual shapes of the tube.
In view of the above, it is an object of the subject invention to provide an apparatus to precisely bend tubular products.
It is another object of the subject invention to provide a bending apparatus that will enable accurate bends despite variations in the metallurgical characteristics of successive tubes.
An additional object of the subject invention is to provide an apparatus that will control successive bends in accordance with variations from a preselected alignment for the bent tubular product.